docs: def check_altitude_units(self, ds):
          """
          If there's a variable named z, it must have units.
  
          @TODO: this is duplicated with check_variable_units
          :param netCDF4.Dataset ds: An open netCDF dataset
          """
          if 'z' in ds.variables:
              msgs = []
              val = 'units' in ds.variables['z'].ncattrs()
              if not val:
                  msgs.append("Variable 'z' has no units attr")
              return Result(BaseCheck.LOW, val, 'Altitude Units', msgs)
  
          return Result(BaseCheck.LOW, (0, 0), 'Altitude Units', ["Dataset has no 'z' variable"])